Drawboring increases the strength of a regular mortise-and-tenon joint by driving a wood pin through a hole in the mortised piece and through a slightly offset hole in the tenon, above. This draws the shoulder of the tenon tight against the face of the mortise. The pin provides additional mechanical strength that keeps the joint tight, even when your joinery isn't a perfect friction fit. Assembly of the joint doesn't require clamps, which makes drawboring particularly attractive when dealing with large assemblies or oddly shaped pieces.
You'll often find drawbore joints in large, timber-style-framed workbenches, and furniture assemblies that experience a lot of racking, such as the rails and feet of a trestle table (photo, above.) or the crest rail of a rocking chair. When finished, the driven pin reveals the beauty of contrasting wood grain that accentuates the forethought taken by the craftsperson to ensure a lifetime of use. Creating this joint requires just a few simple steps, as I'll show.
The hole story
Form your mortises and tenons however you prefer: on the tablesaw, drill press, router table, or by hand. Once you have a fitted joint (remember, it doesn't need to be a perfect friction fit), bore a hole centered on the depth of the mortise. (I drilled a 3⁄8" hole.) You can place the hole closer to the workpiece edge, if you like, as long as you allow sufficient material between the edge and the hole to prevent blow-out while driving the pin. There's no need to drill all the way through the bottom mortise wall (drawing, above), but you can if you want the pin visible on both faces. For a beefy assembly, such as the one shown, I typically drill just deep enough to bury the end of the pin in the far side of the mortise.
Next, dry-fit the joint to locate the offset hole in the tenon, using a centerpunch (or a brad-point drill bit) 1⁄16" smaller than the hole in the mortise (photo, above.). Disassemble the joint and drill on the mark a hole the same diameter as the one in the mortise. Use a backer piece to prevent blow-out as the drill bit exits the tenon. With the joint dry-fit, you can see the offset holes (photo, below.)
Let's put a pin in this
The pins used to drawbore a joint must be flexible, but strong. This requires a straight-grained and stable hardwood—something difficult to find in purchased dowels. Instead, I make my own using scrap stock and a dowel former, a tool with an internal blade that shears the riven wood as you pound it through.
Start by riving a straight-grained white-oak blank about twice as long as the needed pin, shearing off a piece just larger than the desired diameter (3⁄8") (photo, above.)
Then, roughly round and taper the blank (photo, above.) Place the dowel former over a doghole in your bench, then drive the blank through (photo, below.)
To assemble the joint, I spread glue within the mortise and on the tenon, squeeze a bead of glue into the drawbore hole, and lightly coat the pin before driving it in. As you drive the pin through, you'll see the tenon being drawn tightly to the mouth of the mortise (photo, above.) Trim the excess (photo, below), sand the surfaces smooth, and you'll have a mechanically reinforced joint, beautifully accentuated with a contrasting wood.
Source: Veritas dowel former no. 05J6320, Lee Valley, 800-871-8158, leevalley.com.